1. Field of the Invention
This invention relates to telephony systems in general and, more particularly, to protecting subscriber loop interface circuits from overvoltage.
2. Description of the Prior Art
A telephone system, such as an electronic switching system (ESS) or a private branch exchange (PBX), use subscriber line interface circuits (SLICs) to act as an interface between the system and pairs of wires (subscriber loops) to telephone sets. The SLICs provide a variety of well-known functions, some of which are battery feed, ringing, supervision, and message waiting. The functions are generally under the control of a common control in the system.
Early systems utilized relays, inductors, transformers, resistors, capacitors, fuses, and other passive components to implement the above functions. These components are relatively immune from overvoltage stress that may occur, such as lightning strikes or power line cross.
Gradually, as more and more solid state components were added to replace the relays, transformers, etc., the susceptibility of the SLIC to overvoltage stress grew to the point where old protection schemes (e.g., carbon block and gas tube protectors) were insufficient. New schemes (varistors, positive temperature coefficient current limiters, Zener diodes, etc.) were devised to meet the requirements for the newer generations of SLICs.
The push for integrating as much of the SLIC circuitry into one or two chips has again make older overvoltage protection schemes inadequate in many instances. Either the old schemes do not sufficiently protect the integrated SLICs, or the cost and physical size of adequate protection schemes are too much for competitively priced telephone systems to support.
A new approach is needed to provide adequate overvoltage stress protection that is low in cost and physical size. It would be advantageous that the protection be compatible with integrated circuits or be partially integratable with the same circuits that are to be protected.